1. Field of the Invention
The present invention pertains to electrical key switches and, more particularly, to self-encoding key switches for use with data-entry keyboard systems.
2. Discussion of the prior art
Due to the rapid growth of the computer, information handling and data processing industries, the demand for low cost, reliable keyboards and keyboard systems for data-entry is increasing since keyboards provide a natural and convenient interface between human operators and the electronic equipment. To this end, keyboards are familiar input devices for computers and peripheral equipment thereof, calculators, display devices and general communication terminals.
The heart of a keyboard is the individual key switches which are actuated by an operator to generate corresponding coded electrical signals. Key switches for use with keyboards can generally be classified in two major categories; that is, contact and non-contact switches. In contact key switches, a physical contact must be established, such as in reed, cross bar, elastomeric, mercury-filled tube, magnetic repulsion and coil spring, cross-point switches, to name a few. Non-contact switches do not require such physical contact and include Halleffect, capacitive-coupled, saturating magnetic core and photoelectric switches, to name a few. Most contact key switches produce only a simple make or break signal, such as a contact closure, to provide an output voltage or a change of value of some electrical parameter (resistance, capacitance or inductance); and, for this reason, keyboards utilizing such key switches require, in addition to the appropriate number of key switches, electronic circuitry for converting or encoding the key switch signals into desired acceptable codes, for matching the interface characteristics of the associated electronic equipment and for providing N-key rollover to overcome the problem of erroneous signal transmission caused during burst speed key depression when previously struck keys remain depressed while additional keys are depressed. Some non-contact key switches have overcome some of the disadvantages of contact key switches; however, such non-contact key switches have the disadvantages of being relatively expensive and requiring standby power.
Accordingly, it is desirable for key switches to provide the features of N-key rollover and self-encoding; and, further, it is desirable for key switches to provide tactile feedback to the operator in that such tactile feedback essentially duplicates the feel of electric typewriters due to the key force increasing with key displacement until the moment of switch closure whereat the force of pressure drops suddenly. Another desirable feature in key switches is the strobe function which operates to signal the associated electronic equipment that an encoded output has been obtained.
Since key switches and the electronic circuitry for use therewith represent a substantial portion of the cost of keyboards and keyboard systems, many attempts have been made to reduce the electronic circuitry requirements as well as to reduce the cost of the key switches. These attempts in the prior art have not been successful in that attempts to provide self-encoding key switches to reduce circuitry costs have utilized non-contact type switches and, therefore, have increased switch costs while attempts to reduce the cost of key switches have required electronic encoding circuitry and, frequently, additional circuitry to match one type of interface circuit with another thereby increasing electronic circuitry costs. Attempts to develop low cost, self-encoding key switches utilizing a plurality of mechanical contacts have suffered the disadvantages of contact bounce and the inability to economically provide long life, high reliability and N-key rollover.
U.S. Pat. No. 3,903,389 to Faustini discloses a contact key switch overcoming the above disadvantages of the prior art by utilizing mercury to momentarily connect a supply contact with a plurality of encoding contacts and a strobe contact to provide tactile feedback, self-encoding and N-key rollover. While the mercury key switch of the Faustini patent represents a great improvement over the prior art, there is still a need to improve such mercury key switches to facilitate manufacture and assembly of keyboards using the same.